`HashSet.fromList` doesn't use mutation

[sjakobi]

unordered-containers/Data/HashSet/Internal.hs

Lines 448 to 451 in db40495

	-- | \(O(n \min(W, n))\) Construct a set from a list of elements.
	fromList :: (Eq a, Hashable a) => [a] -> HashSet a
	fromList = HashSet . List.foldl' (\ m k -> H.insert k () m) H.empty
	{-# INLINE fromList #-}

Using unsafeInsert instead of insert should speed this up.